Papalos et al., in U.S. Statutory Invention Registration No. H 1,712, disclose radiation-curable compositions containing monoalkyl or mono-aralkyl ether di-acrylates or di-methacrylates of particular ethoxylated and/or propoxylated polyols. These polyol derivatives substituted with hydrophobic ether functionalities are said to constitute UV-curable diluents that can be isolated in excellent yields with minimal losses during the manufacturing process. The UV-curable diluents of Papalos et al. are said to be useful in radiation-curable compositions.
U.S. Pat. No. 4,876,384 issued to Higbie et al., teaches reactive diluents for radiation curable compositions. The diluents are lower alkyl ether acrylates and methacrylates of particular alkoxylated and non-alkoxylated polyols. Examples are mono-methoxy trimethylolpropane diacrylate, mono-methoxy neopentyl glycol monoacrylate and mono-methoxy, ethoxylated neopentyl glycol monoacrylate having an average of about two moles of ethylene oxide.
No mention is made in either of these disclosures of making a coating, adhesive, sealant or elastomer from the compound by combining it with an isocyanate to form a polyurethane.
Polyurethane is typically made by reacting a polyol with an isocyanate. The majority of polyoxyalkylene polyether polyols are polymerized through base catalysis. For example, polyoxypropylene diols are prepared by the base-catalyzed oxypropylation of a difunctional initiator such as propylene glycol. During base-catalyzed oxypropylation, a competing rearrangement of propylene oxide to allyl alcohol continually introduces an unsaturated, monofunctional, oxyalkylatable species into the reactor. The oxyalkylation of this monofunctional species yields allyl-terminated polyoxypropylene monols. The rearrangement is discussed in BLOCK AND GRAFT POLYMERIZATION, Vol. 2, Ceresa, Ed., John Wiley & Sons, pp. 17–21. Unsaturation is measured in accordance with ASTM D-2849-69 “Testing Urethane Foam Polyol Raw Materials,” and expressed as milliequivalents of unsaturation per gram of polyol (meq/g). Due to this continual creation of allyl alcohol and its subsequent oxypropylation, the average functionality of the polyol mixture decreases and the molecular weight distribution broadens. Base-catalyzed polyoxyalkylene polyols contain considerable quantities of lower molecular weight, monofunctional species. In polyoxypropylene diols of 4,000 Dalton (Da) molecular weight, the content of monofunctional species may be between 30 and 40 mole percent. In such cases, the average functionality is lowered to 1.6 to 1.7 from the nominal, or theoretical functionality of 2.0. In addition, base-catalyzed polyols have a high polydispersity (Mw/Mn) due to the presence of the substantial, low molecular weight fractions.
Lowering unsaturation and the attendant high monol fraction in polyoxypropylene polyols has been touted as a means of providing polyurethane compositions with improved properties. Often, formulations must be chosen to balance conflicting properties. For example, increases in tensile strength are often accompanied by a decrease in elongation.
Therefore, a need exists in the art for UV-curable polyurethane compositions having a low modulus, high elongation, high tensile strength and solvent resistance that are suitable for use in or as coatings, adhesives, sealants, elastomers and the like.